Temperature and electron density dependence of spin relaxation in GaAs/AlGaAs quantum well
© Han et al; licensee Springer. 2011
Received: 12 September 2010
Accepted: 12 January 2011
Published: 12 January 2011
Temperature and carrier density-dependent spin dynamics for GaAs/AlGaAs quantum wells (QWs) with different structural symmetries have been studied by using time-resolved Kerr rotation technique. The spin relaxation time is measured to be much longer for the symmetrically designed GaAs QW comparing with the asymmetrical one, indicating the strong influence of Rashba spin-orbit coupling on spin relaxation. D'yakonov-Perel' mechanism has been revealed to be the dominant contribution for spin relaxation in GaAs/AlGaAs QWs. The spin relaxation time exhibits non-monotonic-dependent behavior on both temperature and photo-excited carrier density, revealing the important role of non-monotonic temperature and density dependence of electron-electron Coulomb scattering. Our experimental observations demonstrate good agreement with recently developed spin relaxation theory based on microscopic kinetic spin Bloch equation approach.
Spin dynamics and the related physics in semiconductors have drawn much attention in the past years because of its importance to realize novel spin-electronic devices . In recent years, electron spin relaxation in many types of materials, especially in low dimensional III-V group semiconductor heterostructures, has been studied extensively both theoretically and experimentally . The relevant spin relaxation mechanisms, such as the Elliott-Yafet, Bir-Aranov-Pikus (BAP), and D'yakonov-Perel' (DP) mechanisms as well as hyperfine interactions, have been well established to describe spin relaxation and dephasing dynamics. However, the relative importance of these mechanisms strongly depends on material design and temperature as well as carrier concentration and so on. Previous investigations in literature show that the BAP mechanism dominates the spin relaxation at low temperatures for bulk GaAs [2, 3] and GaAs/AlGaAs quantum wells (QWs) [4, 5], whereas DP mechanism dominates spin relaxation in other regimes. However, recent reexaminations using the microscopic kinetic spin Bloch equation approach [6–9] have revealed that the BAP mechanism is much less important than DP mechanism for intrinsic III-V group semiconductors, even at low temperatures. The DP mechanism resulting from spin-orbit coupling in systems lacking inversion symmetry, such as zinc-blende structure or asymmetric confining potentials in QWs, has a spin relaxation rate inversely related to the momentum scattering rate . Electron spin relaxation in GaAs QWs has been experimentally studied through temperature [10, 11] and QW width dependence [10–12], and DP mechanism has been revealed to dominate spin relaxation in intrinsic QWs at high temperatures . The oscillatory spin dynamics study for two-dimensional electron gas (2DEG) at low temperatures also demonstrated the dominance of DP mechanism in the weak momentum scattering regime . The observed enhancement of spin relaxation time resulting from electron-electron scattering in n-doped GaAs/AlGaAs QW agrees with DP mechanism governed by electron-electron scattering as well [14–17]. The experimental observation of electron spin relaxation time maximum for temperature-dependent study in a high-mobility GaAs/AlGaAs 2DEG has also revealed the importance of electron-electron Coulomb scattering .
The spin-orbit (SO) coupling leads to a strong momentum-dependent mixing of spin and orbital-momentum eigenstates, so that scattering processes change spin and orbital angular momentum, and therefore contribute to spin relaxation accordingly [19–21]. For electrons in two-dimensional semiconductor heterostructures or QWs, the Rashba SO coupling due to structure inversion asymmetry and the Dresselhaus SO coupling due to bulk inversion asymmetry in the compounds cause electron spin relaxation and decoherence through spin precession of carriers with finite crystal momentum k in the effective k-dependent crystal magnetic field of an inversion-asymmetric material. Therefore, spin relaxation and decoherence studies in semiconductors have revealed important physics of SO coupling. Since carrier spin relaxation is related to several competing mechanisms and particularly different materials and structural designs, different SO coupling is involved to spin relaxation processes. Thus, the experimental investigation of spin relaxation and its dependence on temperature and carrier density have been found to vary widely between different samples. In this study, we have designed two GaAs/AlGaAs QWs with different structural symmetries. The spin relaxation time is measured to be much shorter for the asymmetrically designed GaAs/AlGaAs QW comparing with the symmetrical one, indicating the strong effect of Rashba SO coupling on spin relaxation. The comprehensive studies of temperature and carrier density dependence of spin relaxation time for both samples have revealed that electron spin relaxation in GaAs/AlGaAs QWs is governed mainly by DP mechanism in the entire temperature regime. The spin relaxation time exhibits non-monotonic behavior for both temperature and photo-excited carrier density dependence, revealing the important role of non-monotonic temperature and density dependence of electron-electron Coulomb scattering. Our experimental observations demonstrate good agreement with recently developed spin relaxation theory based on microscopic kinetic spin Bloch equation approach [6–9].
In our time-resolved magneto-Kerr rotation measurement, a Ti:Sapphire laser system (Chameleon Ultra II, Coherent Inc., USA) provided 150 fs pulses with repetition rate of 80 MHz. The pump beam with central wavelength ranging from 770 to 860 nm was incident normal to the sample, while probe beam was at an angle of about 30° to the surface normal. The polarization of the pump beam was adjusted to be circularly polarized and the probe beam was linearly polarized. The sample was mounted within a Janis closed-cycle optical cryostat, which is located in-between two poles of an electromagnet. After reflection from sample, the Kerr rotation signal was detected by a sensitive optical bridge and lock-in amplifier. The photoluminescence (PL) measurements have been first performed at wide temperature range to check the sample's quality and identify the band-edge energies for the specially designed samples.
Results and discussion
When temperature further increases, electron-phonon scattering will then strengthen and become comparable to electron-electron scattering, eventually dominate the spin relaxation process; thus, spin relaxation time decreases with further rising temperatures. As a result, spin relaxation time shows a maximum. Considering the total electron density, n e is the sum of optically excited carrier density and doping density (assuming fully ionized Si doping), the peak of temperature-dependent spin relaxation time is calculated to appear at about 59 and 140 K for the symmetric and asymmetric GaAs/AlGaAs QW samples under optically pumped electron density of 1.15 × 1011 cm-2, respectively. This, however, only agrees with the observed peak position for the symmetric sample. The inconsistence for the asymmetric sample may result from the uncertainty of the actual electron density.
In conclusion, the temperature and carrier density-dependent studies of spin relaxation time for modulation-doped GaAs/AlGaAs QWs have demonstrated a good agreement with recently developed spin relaxation theory based on microscopic kinetic spin Bloch equation approach. The spin relaxation time is measured to be much longer for the symmetrically designed GaAs QW comparing with the asymmetrical one, indicating the strong influence of Rashba SO coupling on spin relaxation. DP mechanism has been revealed to dominate spin relaxation for n-modulation-doped GaAs QWs in the entire temperature regime. Our experimental results provide further fundamental understanding of spin dynamics in modulation-doped heterostructures toward potential semiconductor spintronics application based on GaAs/AlGaAs material systems.
two-dimensional electron gas.
This study was supported by the National Basic Research Program of China under Grant Nos. 2011CB922200 and 2007CB924904; the National Natural Science Foundation of China under Grant Nos. 10974195 and 10734060.
- Dyakonov MI: Spin Physics in Semiconductors. Berlin: Springer-Verlag; 2008.View Article
- Lai TS, Liu XD, Xu HH, Jiao ZX, Wen JH, Lin WZ: Temperature dependence of electron-spin coherence in intrinsic bulk GaAs. Appl Phys Lett 2006, 88: 192106. 10.1063/1.2202754View Article
- Amo A, Viña L, Lugli P, Tejedor C, Toropov AI, Zhuravlev KS: Pauli blockade of the electron spin flip in bulk GaAs. Phys Rev B 2007, 75: 085202. 10.1103/PhysRevB.75.085202View Article
- Wagner J, Schneider H, Richards D, Fischer A, Ploog K: Observation of extremely long electron-spin-relaxation times in p-type delta-doped GaAs/Al x Ga 1-x As double heterostructures. Phys Rev B 1993, 47: 4786. 10.1103/PhysRevB.47.4786View Article
- Gotoh H, Ando H, Sogawa T, Kamada H, Kagawa T, Iwamura H: Effect of electron-hole interaction on electron spin relaxation in GaAs/AlGaAs quantum wells at room temperature. J Appl Phys 2000, 87: 3394. 10.1063/1.372356View Article
- Zhou J, Wu MW: Spin relaxation due to the Bir-Aronov-Pikus mechanism in intrinsic and p-type GaAs quantum wells from a fully microscopic approach. Phys Rev B 2008, 77: 075318. 10.1103/PhysRevB.77.075318View Article
- Jiang JH, Wu MW: Electron-spin relaxation in bulk III-V semiconductors from a fully microscopic kinetic spin Bloch equation approach. Phys Rev B 2009, 79: 125206. 10.1103/PhysRevB.79.125206View Article
- Jiang JH, Cheng JL, Wu MW: Spin relaxation in n -type GaAs quantum wells from a fully microscopic approach. Phys Rev B 2007, 75: 045305. 10.1103/PhysRevB.75.045305View Article
- Wu MW, Jiang JH, Weng MQ: Spin dynamics in semiconductors. Phys Rep 2010, 493: 61. 10.1016/j.physrep.2010.04.002View Article
- Malinowski A, Britton RS, Grevatt T, Harley RT, Ritchie DA, Simmons MY: Spin relaxation in GaAs/Al x Ga 1-x As quantum wells. Phys Rev B 2000, 62: 13034. 10.1103/PhysRevB.62.13034View Article
- Ohno Y, Terauchi R, Adachi T, Matsukura F, Ohno H: Spin relaxation in GaAs (110) quantum wells. Phys Rev Lett 1999, 83: 4196. 10.1103/PhysRevLett.83.4196View Article
- Britton RS, Grevatt T, Malinowski A, Harley RT, Perozzo P, Cameron AR, Miller A: Room temperature spin relaxation in GaAs/AlGaAs multiple quantum wells. Appl Phys Lett 1998, 73: 2140. 10.1063/1.122403View Article
- Leyland WJH, Harley RT, Henini M, Shields AJ, Farrer I, Ritchie DA: Oscillatory Dyakonov-Perel spin dynamics in two-dimensional electron gases. Phys Rev B 2007, 76: 195305. 10.1103/PhysRevB.76.195305View Article
- Leyland WJH, John GH, Harley RT, Glazov MM, Ivchenko EL, Ritchie DA, Farrer I, Shields AJ, Henini M: Enhanced spin-relaxation time due to electron-electron scattering in semiconductor quantum wells. Phys Rev B 2007, 75: 165309. 10.1103/PhysRevB.75.165309View Article
- Wu MW, Ning CZ: A novel mechanism for spin dephasing due to spin-conserving scatterings. Eur Phys J B 2000, 18: 373. 10.1007/s100510070021View Article
- Wu MW, Metiu H: Kinetics of spin coherence of electrons in an undoped semiconductor quantum well. Phys Rev B 2000, 61: 2945. 10.1103/PhysRevB.61.2945View Article
- Wu MW: Spin dephasing induced by inhomogeneous broadening in D'yakonov-Perel' effect in a n -doped GaAs quantum well. J Phys Soc Jpn 2001, 70: 2195. 10.1143/JPSJ.70.2195View Article
- Ruan XZ, Luo HH, Yang J, Xu ZY, Umansky V: Effect of electron-electron scattering on spin dephasing in a high-mobility low-density two-dimensional electron gas. Phys Rev B 2008, 77: 193307. 10.1103/PhysRevB.77.193307View Article
- Sham LJ: Spin relaxation in semiconductor quantum wells. J Phys Condens Matter 1993, 5: A51. 10.1088/0953-8984/5/33A/005View Article
- Averkiev NS, Golub LE, Willander M: Spin relaxation anisotropy in two-dimensional semiconductor systems. J Phys Condens Matter 2002, 14: R271. 10.1088/0953-8984/14/12/202View Article
- Pikus GE, Titkov AN: Optical Orientation. Edited by: Meier F, Zakharchenya BP. Amsterdam: North-Holland; 1984:73–131.View Article
- Teng LH, Zhang P, Lai TS, Wu MW: Density dependence of spin relaxation in GaAs quantum well at room temperature. EuroPhys Lett 2008, 84: 27006. 10.1209/0295-5075/84/27006View Article
- Ma H, Jin ZM, Ma GH, Liu WM, Tang SH: Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature. Appl Phys Lett 2009, 94: 241112. 10.1063/1.3155428View Article
- Jiang JH, Wu MW: Comment on "Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature". Appl Phys Lett 2010, 96: 136101. 10.1063/1.3371817View Article
- Krauß M, Schneider HC, Bratschitsch R, Chen Z, Cundiff ST: Ultrafast spin dynamics in optically excited bulk GaAs at low temperatures. Phys Rev B 2010, 81: 035213.View Article
- Shen K: A Peak in Density Dependence of Electron Spin Relaxation Time in n -Type Bulk GaAs in the Metallic Regime. Chin Phys Lett 2009, 26: 067201. 10.1088/0256-307X/26/6/067201View Article
- Oertel S, Hübner J, Oestreich M: High temperature electron spin relaxation in bulk GaAs. Appl Phys Lett 2008, 93: 132112. 10.1063/1.2993344View Article
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